Satellite communication is used in a wide variety of systems and applications. For example, satellite communication can be used for transmitting digital video or for providing high-speed Internet access to mobile terminals. Some known satellite communication systems transmit signals jointly via two satellite transponders. For example, U.S. Pat. No. 6,694,137, whose disclosure is incorporated herein by reference, describes a communication system that causes a pair of satellite transponders produce a circularly-polarized signal. Uplink signals are provided to dedicated paired transponders on geostationary satellites. The uplink signals contain broadcast data and correction factors that maintain a high degree of purity in the quality of the circular polarization of the signal received on the ground.